1. Field of the Invention
The present invention relates to formation of an image on a translucent sheet.
2. Description of the Related Art
In recent years, an image forming apparatus called an MFP (Multi Function Printer) including the functions as a printer, a scanner, a facsimile, etc., is widely used. Constructing an image forming system in which a so-called printing control apparatus is connected to such an image forming apparatus is also performed widely. The printing control apparatus is an apparatus that performs various kinds of image processing related to print job processing, performs various kinds of setting processing for the image forming apparatus, and gives instructions to perform printing, and is also called a DFE (Digital Front End).
The purpose of connecting the DEF to the MFP is to improve the functions that are provided by the MFP and to provide the functions unique to the DFE. An example of improvement in function is to improve the processing performance of a print job. By causing the DFE to interpret the page description language (PDL) and to generate bitmap data, and by causing the MFP to perform processing to print the generated bitmap data, it is possible to efficiently perform print job processing in a brief time compared to the case where the MFP alone performs all the processing. The functions unique to the DFE include page edition and imposition of a print job, image processing setting, image forming processing in accordance with the kind of a sheet on which printing is performed, etc. There are miscellaneous kinds of printing sheets on which an image can be formed by the MFP, including not only the copy sheet (plain sheet) but also the coated sheet, the label sheet, and the index sheet. Such a special sheet is used in order to add a value to a printed product. The DFE and the MFP collaborate in changing parameters of image processing in accordance with the kind of a printing sheet, in changing the layout of an image, etc.
Some of the printing sheets have a tendency to cause a large amount of light to pass through and characteristics that an image printed on the back side or an image printed on the next page shows through. An example of such a printing sheet is an OHP film, a polypropylene film, or tracing paper. Such a sheet causes light to penetrate therethrough by selecting a material and reducing the thickness of the sheet. The degree of transparency differs from sheet to sheet and there is a sheet whose degree of transparency is high and the back side shows through as it is or a semitransparent sheet whose degree of transparency is reduced and which shows a unique feeling when an image on the back side is seen through. In the present specification, the printing sheet having optical translucency is called a translucent printing medium. In the case where an image is formed on a translucent printing medium, an image that is printed on the back side or on the next page shows through, and therefore, various image forming methods in accordance with purposes have been proposed. In particular, because an image that is printed on the back side can be seen from the front side, there exists a technique to improve quality of a printed product by performing mirror-image conversion on all or part of the image data to be printed on a translucent printing medium and performing printing on the back side. For example, Japanese Patent Laid-Open No. 2011-112681 has described an image forming method that is effective in the case where the back side of printed matter obtained by using a translucent printing medium is irradiated with light from a light source for an advertisement use. The technique described in Japanese Patent Laid-Open No. 2011-112681 is a technique to extract only the high-density portions of the front side image and to print the extracted image also from the back side after performing mirror-image conversion on the extracted image in order to keep the gradation properties at low densities while maintaining the high density of the image that is formed.
However, the technique described in Patent document 1 premises that an image that is printed on the back side shows through, but whether the image that is printed on the back side actually shows through the front side is not determined. The translucent printing media are manufactured by many makers and the number of the kinds of the translucent printing media is large. Further, whether or not an image shows through a translucent printing medium depends also on the image forming apparatus that is used and the density of the color material. Furthermore, the determination of whether or not an image shows through may differ depending on the viewpoint of a user. Because of this, it is necessary for a user to check how an image shows through by actually performing trial printing and to repeatedly perform trial printing until an expected printed product is obtained (until an image shows through as expected).